[till]

Is there a simple way to modify the script to work on a temporary copy? I would rather not deactivate the traffic statistics, customers would probably complain.

I checked the mail logs script. ISPConfig works already on a temporary copy and not on the original mail logfile.

Also the mail_logs.php script won't compress old logs, this seems a bit wasteful. I noticed my maillogs for this machine are above 5 GB since about may I do monitor so i would've caught this before it became a problem, but i tend to assume logs are rotated correctly.

ISPConfig does not rotate logs, its just analyzes a temporary copy. You should check your log rotate settings to ensure that the mail log is compressed.